Cellular Adds Complexity to the RF Design Process
Jul 1, 2012
July 2012 - Telit 2 Market Magazine - A major difference between a cellular M2M and an RF design, is working with network providers that offer different coverage areas and rate plans. The amount of data throughput required by the application helps determine if 2G, 3G, 4G technology is needed. Identifying the specific cellular protocol (GSM, CDMA, HSPA, etc) narrows down the selection of available networks. A good cellular design will support multiple networks and allow for different protocols depending on deployment location.
Most cellular designs will use a module rather than a chip due to the significant effort and cost required to certify devices on a cellular network. It may make sense to implement chip-level Bluetooth, Wi-Fi or Zigbee designs for annual unit volumes in the hundred thousands, but the economics of a discrete cellular design don’t happen until volumes reach close to a million units. In addition to cellular modules, which are typically certified on networks as a just that, there are fully enclosed modem / terminal products certified as end devices. This allows customers to use a modem without any additional certification. A modem solution is recommended for products requiring less than two thousand cellular devices a year.
Certification for cellular designs is more complex than non-cellular RF designs. In addition to FCC, the product must be certified on a specific network. Costs associated with this certification make it important to select a module and antenna partner that are familiar with this process and are willing to assist throughout the design and certification stages. It’s key to select a module that is pre-certified by the chosen network. GSM-based networks require that end products using pre-certified modules pass PTCRB certification and then network approval. CDMA requires network certification which can be done at an independent certification facility or at one of the network’s facilities. The costs for PTCRB and network approval can become onerous if multiple attempts are required to bring all specs within tolerance. The networks have different test limits, so certifying on one network does not guarantee that the product will be in the limits of another.
The main components in an M2M architecture are the cellular radio, antenna, host controller, and power management. Although some cellular modules have embedded processors with scripting languages to allow limited operation without an external host, most applications are better served by an external host controller. This gives the design more flexibility adding the I/O and peripherals offered by a host controller in addition to all features of the cellular module. The controller interfaces to the cellular module with AT commands over a UART or SPI connection (USB is becoming common on 3G modules). A cellular module may require up to 2 Amps of peak current, so power ICs use by other RF designs may not be adequate for cellular.
During part selection it’s important to choose a cellular module partner rather than opting for a module based solely on price and a datasheet. A good module partner will have various network approved modules to support the different cellular technologies and be able to help with design reviews. A module partner who has developed a common software command set for their modules can save significant time with future designs. The module providers who have well documented hardware and software user manuals are much more capable of supporting different applications. A stable development kit that supports multiple modules will help accelerate the design cycle.
While it’s feasible to select an ISM or 2.4GHz antenna based on its specs, choosing a cellular antenna should be based on the capabilities of the antenna partner. Antenna partners should not only help select the right antenna, but should also participate in design reviews and be able to run pre-certification tests. Pre-cert testing at an antenna partner’s facility can save substantial time and money during a costly certification process.
David Beck - Director of Technical Marketing - Symmetry Electronics